Neutron source with beam shaping apparatus for radiography

ABSTRACT

A neutron generator has a neutron source generating an ion beam bombarding a Titanium target having a first diameter, the target embedded in a pre-moderator, emitting fast neutrons isotropically, a portion of the fast neutrons moderated in passing through the pre-moderator and exiting through a lowermost surface of the pre-moderator, and a plate of moderating material abutting the lowermost surface of the pre-moderator, the plate having an opening therethrough in a shape of a truncated cone with an axis aligning with direction of the ion beam, a depth, a major diameter of at the upper surface of the plate and a minor diameter at the lower surface of the plate, the opening forming a funnel through which neutrons pass. Neutrons enter the funnel and are to exit through the minor diameter of the funnel, providing a neutron beam with a spot size useful for neutron radiography.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention is in the technical area of apparatus and methods forradiography and pertains more particularly to accomplishing radiographyby focusing a neutron beam coming from fusion neutron generator.

2. Description of Related Art

Neutron radiography is a non-destructive imaging technology that usesthermal neutrons to image objects. Unlike x-rays, neutrons primarilyinteract with atomic nuclei. Therefore, the attenuation pattern ofthermal neutrons is quite different and provides contrasts between othermaterials. The attenuation of x-rays continuously increases with theatomic number, while there is no significant correlation betweenattenuation of thermal neutrons and the atomic number. For example, highattenuation of thermal neutrons in hydrocarbons will contrast with thatof other materials such as aluminum or steel, which can be penetrated byneutrons. Therefore, thermal neutron radiography is widely used in caseslike inspection of hydrocarbons and plastics embedded in metals.

Thermal neutron radiography is performed in conventional art usingnuclear reactors which have highly divergent sources of neutrons withadditional spurious components of fast neutrons and gamma rays, whichblur and deteriorate quality of an image. Thus, it is necessary toreduce these contaminating components and collimate the neutrons togenerate a sharp radiograph with high resolution. Thus the use of aneutron collimator and both spatial and energy filters becomes importantfor using thermal neutrons for radiography. The choice of thesecomponents along with the neutron source becomes critical.

Indeed, thermal neutron radiography could become a relatively maturetesting technique if more inexpensive and compact neutron sources becomeavailable. Other industrial uses may be found in areas involvingexplosives, plastics and other low atomic number materials contained inmetal parts. Neutron Radiography for non-destructive testing (NDT)requires new, relatively inexpensive source.

Low Voltage, Fusion neutron generators (LVFGs) permit a long-lived,easily moderated neutron source to be available for radiography, andother applications. However, compact neutron generators using the DDfusion reaction have emission that is isotropic and not directional,and, hence, directing most of the neutrons produced at a Ti target isnot easily achieved. Further, most moderation processes result inundesirable components such as gamma and higher energy neutrons.Moderating the fast neutrons to thermal energies also results inreduction of desired thermal neutron flux and brightness. Obtainingdirectional, high fluxes of thermal neutrons on a sample has beendifficult without the extensive losses of neutrons and an enlargedthermal neutron source.

Unlike reactors, the LVFG has qualities that can compensate for thisissue, such as small source size, high neutron brightness, and low fastneutron energy (2.5 MeV). Small source size allows for easier collectionand moderation of fast neutrons into either thermal or epithermalneutrons, thereby increasing neutron flux.

What is clearly needed in the art is a modular, relatively small LVFGthat may be joined to collimating apparatus to produce a substantiallyfocused beam of thermal neutrons, interfaced to imaging apparatus toprovide an efficient and effective radiography.

BRIEF SUMMARY OF THE INVENTION

In an embodiment of the invention a neutron radiography generator isprovided, comprising a modular neutron source generating an ion beam ina first direction, the ion beam bombarding a Titanium target having asurface comprising a first diameter, the target embedded in apre-moderator having a lowermost surface, emitting fast neutronsisotropically, a portion of the fast neutrons moderated in passingthrough the pre-moderator and exiting through the lowermost surface ofthe pre-moderator, and a plate of moderating material abutting thelowermost surface of the pre-moderator, the plate having an uppersurface, a lower surface, a thickness, and an opening therethrough in ashape of a truncated cone with an axis aligning with the first directionof the ion beam, a depth, a major diameter of at the upper surface ofthe plate and a minor diameter at the lower surface of the plate, theopening forming a funnel through which neutrons pass. Neutrons exitingthe pre-moderator through the lowermost surface thereof enter the funneland are collimated through the funnel to exit through the minor diameterof the funnel, providing a neutron beam with a spot size useful forneutron radiography.

In one embodiment the thickness of the plate of moderating material isgreater than the depth of the funnel, further comprising a sleeve ofhigh-density neutron reflecting material having in inside diameter equalto the major diameter of the truncated cone shape of the funnel, thesleeve lining a circular hole in the plate of moderating material, thehole having a first depth from the upper surface to the major diameterof the truncated cone shape of the funnel, such that the depth of thecircular hole and the depth of the funnel equals the thickness of theplate, and the inside diameter of the sleeve of reflecting materialmeets the major diameter of the truncated cone shape of the funnel.Also, in one embodiment the generator further comprises a disk ofBismuth having a diameter equal to the inside diameter of the sleeve anda length less than the first depth of the hole, and a solid cylinder ofsapphire crystal having a diameter equal to the inside diameter of thesleeve and a length such that the length of the bismuth disk and thesapphire cylinder equal the length of the hole from the upper surface ofthe plate of moderating material to the major diameter of the funnel,the bismuth disk and sapphire crystal attenuating gamma radiation andattenuating fast neutrons. In one embodiment the neutron radiographygenerator further comprises a layer of shielding material cladding theeouter surfaces of the generator including the lower surface of the plateof moderating material, with a hole through the shielding material atand equal in diameter to the minor diameter of the funnel.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a transverse cross section of a modular neutron generator inan embodiment of the present invention.

FIG. 1B is a longitudinal cross section of the modular generator of FIG.1A.

FIG. 2 is a diagram showing “geometrical un-sharpness” or blur inradiography using neutron sources.

FIG. 3 is a cross section view of the LVFG, its pre-moderator, and aconvergent funnel for a small area source in an embodiment of theinvention.

FIG. 4A illustrates thermal neutron yield (n/sec-cm²) as a function ofdistance x across an axis of the convergent funnel of a simplebeam-shaping assembly (BSA) in an embodiment of the invention.

FIG. 4B illustrates measured thermal neutron yield (n/sec-cm²) as afunction of distance x across the axis of the convergent funnel of thesimple BSA (Slab with funnel) and for the case of no funnel and themoderator with the thickness L₁+L₂=9.5 cm,

FIG. 5 is a cross sectional view of components required for producing aknife edge image for a computer simulation in an embodiment of theinvention.

FIG. 6 illustrates a computer simulation of a knife edge made ofGadolinium (Gd) for fast neutrons (E>0.5 eV) and thermal neutrons (E<0.5eV) in an embodiment of the invention.

FIG. 7A shows image flux as a function of x, the transverse distanceacross a detector array in an embodiment of the invention.

FIG. 7B shows image flux as a function of x, the transverse distanceacross the detector array in an embodiment of the invention.

FIG. 8 is image resolution (mm) of the knife edge as a function of L,the distance between the end of BSA and the knife edge in an embodimentof the invention.

FIG. 9 shows estimated time (sec) it takes to measure the knife edge asa function of L, the distance between the end of BSA and the knife edgein an embodiment of the invention.

FIG. 10A is a longitudinal cross-sectional of the modular generator witha BSA composed of Bismuth foil, a funnel, an exit aperture, and asapphire crystal with a reflecting graphite sleeve in an embodiment ofthe invention.

FIG. 10B is a transverse cross-section view of the modular generator ofFIG. 10A taken along an axis of the acceleration chamber, and along theaxis of a turbo vacuum pump in an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A and 1B are cross-sectional views of a modular neutron generator118 known to the inventors designed to produce a substantial flux ofthermal neutrons. Modular generator 118 comprises a pre-moderator 108that is made of material known to moderate energy of fast neutrons tothermal energies. In most embodiments for thermal neutron production thepre-moderator may be a shaped, solid block of material such asHigh-Density Polyethylene (HDPE) or Ultra High Molecular Weight (UHMW)polyethylene. Selection of material and its thickness is determined atleast in part by the desired neutron spectrum (e.g. thermal orepithermal) and desired neutron yield.

Modular generator 118 has four important elements in this example: (1) adeuterium ion source 102, (2) an acceleration chamber 100, through whichdeuterium ions 104 are accelerated, and (3) a titanium target 106 thatis bombarded by the deuterium ions to produce high-energy neutrons 110.Deuterium ion source 102 has an attached microwave source 160 in theimplementation, and microwave slug tuners 172. In operation Deuteriumgas is leaked slowly into a plasma ion chamber 174 at an upper end ofthe acceleration chamber, where microwave energy ionizes the gas,creating deuterium D⁺ ions 104. The gas is ionized by microwave energy,and Deuterium ions (D⁺) 104 are created and accelerated through an ionextraction iris 138 into acceleration chamber 100, and through anelectron suppression shroud 180 which deflects back-streaming electronsfrom being accelerated back into the plasma source, which could damagethe apparatus. Electrons are created by collisions of the D⁺ ions in thedeuterium gas that are being created in the acceleration chamber.

The deuterium ions are positively charged, and target 106 is negativelycharged to a level of from 120 kV to 220 kV, and the D⁺ ions arestrongly attracted to negatively biased titanium (Ti) target 106.Acceleration chamber 100 is connected to a turbo vacuum pump 124 thatprovides a modest vacuum in one embodiment of about 10⁻⁶ Torr,minimizing scattering of the D⁺ ions as they travel from extraction iris138 to target 106. Titanium target 106 is positioned in a cavity 181 atthe bottom of the chamber, the cavity formed in the pre-moderatormaterial. Pre-moderator 108 has a passage for a high voltage cable andfluid cooling channels to and from the target. Pre-moderator 108 acts asa high-voltage insulator and as a mechanical support for the target at ahigh negative bias. When in operation the D⁺ ions in the ion beam areattracted to the titanium target 106, where fast neutrons are producedin a resulting DD fusion reaction.

A major issue for fusion sources using the Deuterium-Deuterium (D-D)reaction to produce fast neutrons that must be moderated to thermalneutron energies is that fast and epithermal neutrons as well as highenergy gamma emission are usually part of the moderation of the fastneutrons to thermal energies. These components can accompany the thermalneutrons penetrating the absorbent material of the iris and mayeffectively increase the aperture size D if the extraneous radiation canpenetrate the iris materials, blurring the desired image.

In large reactors, thermal neutrons have been obtained which havemixtures of thermal, epithermal, and fast neutrons along with gamma andx-rays. Applications such as neutron radiography and radiotherapyusually require the neutron energy to be confined to single neutronenergy bands without x-ray or gamma components. There need be methods toeliminate the unwanted radiation components.

Modular DD fusion generator 118 in embodiments of the present inventionuses a small titanium target (e.g. a 5-7 cm diameter disk of titaniumbacked by water-cooled copper fins) to produce neutrons. FIGS. 1A and 1Bare cross-sectional drawings of a modular neutron generator to producemaximum thermal neutrons. The target is supported directly on thepre-moderator, which is an integral part of the apparatus in thisimplementation. The Ti target may be attached with fasteners to thepre-moderator block and may be sealed to the block with an O-ring.Targets in embodiments of the invention can be easily manually removedand replaced. They also have a long lifetime and have been tested forover 4000 hours with no failures.

In the following descriptions reference is made to accompanying drawingsthat form a part of the disclosure and teaching of the presentinvention, and which illustrate specific embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural changes may be made withoutdeparting from the scope of the present invention.

The accelerator structure in embodiments of the invention is compact andincludes a pre-moderator 108 that adds only about 4-5 cm of High-DensityPolyethylene (HDPE) or 15-20 cm of polytetrafluoroethylene (PTFE) Teflonto produce a first stage of neutron beam tailoring in embodiments of theinvention. The pre-moderator in these embodiments is an integral part ofeach modular generator, as is taught below with reference to severalfigures. Other short-length attachments are added to the pre-moderatorto further improve the neutron beam in beam purity, size and shape,making the modular neutron generator a highly versatile source ofneutrons. A primary application for the unique apparatus taught in thisapplication is thermal neutron radiography, which requires a smallsource size, high neutron yield (n/cm²) and high beam purity. Highthermal neutron beam purity is achieved in embodiments of the inventionby minimizing other neutron and photon components that may be introducedduring the DD fusion process and moderation of the 2.5 MeV neutrons tothermal energies. The filtering process is accomplished using neutronfilters; both “low pass” and “high pass” filters. To maximize theresulting neutron flux and minimize the neutron source size, thesefilters and collimators are minimalized in length and proximity to theneutron generator. This results in a highly compact and useful neutronsource for many applications.

As is known for most radiation sources, a small source size is requiredfor image clarity and sharpness. “Geometric un-sharpness” or “blur”refers to loss of image detail caused by the finite size of the sourcediameter. This is true of sources of radiation such as x-ray tubes,where an electron beam of diameter D strikes the anode (e.g. tungstentarget) of the x-ray tube, producing a source size of diameter D. Inother sources of radiation used for radiographic imaging (examplesynchrotron radiators or plasma pinch sources of x-rays), the spot orsource size D can be defined by the either a slit or metal aperture thatdefines where the x rays or neutrons are being emitted. In the presentinvention, the aperture is defined by an exit aperture D2 of a funnel orconical aperture.

The source or spot size can result in “geometric un-sharpness”, “blur”,or the loss of image detail caused by the finite size of the neutronemission size of diameter D₂. In neutron sources the spot size D₂ isdefined by an exit aperture after the neutrons have been moderated tothermal neutron energies. The convergent aperture is defined by an exitaperture which can be made of different materials, such as HDPE andgraphite, which result in collection and collimation of the thermalneutrons.

As shown in FIG. 2 with simple geometry, minimum blur (B_(o), 226) maybe achieved if the neutron source-to-detector distance L, 240, and thesource-emission diameter, D₂, 228, results in a small D₂/L. This permitsa resolution of Δx(D₂/L) for an object that is Δx thick 242. For higherresolution (less blur B_(o), 226) this relationship requires the sourcediameter D₂ to be small as possible and as far from the detectordistance, L 240, as possible. Placing an aperture over the neutronsource that limits source size D₂ will reduce the flux density(N/sec-cm²) where N is the number of neutrons per sec that pass throughthe aperture to the detector. The optimum is to reduce D₂ without lossof neutrons. Focusing optics achieve this for photons, but for neutronsin the thermal energy range, this is difficult in a short distance L,without loss of neutrons from an ever-expanding neutron beam.

Thermal neutron collection can be achieved with a funnel 222 (FIG. 3) toboth collect and channel neutrons into a small spot size D₂ withincreased thermal flux at the exit D₂ of the funnel. The compact DDfusion source with a short (4-5 cm) moderator slows the fast 2.5 MeVneutrons to thermal energies in a short distance from the fast neutronsource (the titanium target). The neutrons are then collected by arelatively short (e.g. L₂=3 to 4 cm) funnel shape formed into a slab ofHDPE. Both moderation and scattering continue to occur along the funnellength L. Simulations show that this results in an increase in flux 2 to3 times and spot sizes of 1 to 3 cm in diameter depending on thegeometry of the cone and size of the fast neutron emitter (Ti target).

Compactness of the DD fusion generator, and shortness of thepre-moderator to produce and collect thermal neutrons also allow for theuse of other devices in the neutron beam. These devices include shortlengths of sapphire crystals and bismuth which can reduce fast neutronsand gamma emission in the neutron beam, thus cleaning up the beam andachieving a relatively pure beam of thermal neutrons.

In fusion devices, such as the LVFG in the present invention, the numberof neutrons is limited. The use of a compact fusion generator withrelatively small spot sources of neutrons permits neutron filters toalso be compact and close together. The modular generator combinesmultiple functions that were separate functions in the prior art. Theseintegrated functions include both neutron production and neutron energyband selection. This method shortens the overall length of the deviceand ensures high fluxes.

As was shown in FIG. 2, a critical requirement for image resolution is asmall diameter source size of high neutron yield. FIG. 3 is a crosssection of the neutron generator. In one embodiment a simple BeamShaping Apparatus (BSA) in the form of funnel 222 is added, which mayproduce a small source size for radiography. In one embodiment theconvergent funnel has a base diameter D₁ of 6 cm and exit opening D₂ of1.5 cm. The funnel collects thermal neutrons being emitted bypre-moderator 108. The section of FIG. 3 is taken along an axis of anacceleration chamber 100 for ion beam generation and at a right angle tothe axis of a turbo vacuum pump, 124, that is part of the modulargenerator.

In one embodiment an input aperture D₁ (252) of the funnel 222 is placedapproximately at the L₁=5.5 cm away from the titanium target 106 whosediameter is 6 cm. This location is where the thermal neutron flux hasbeen shown to be maximum and where collecting the thermal neutronsmaximizes the neutrons at the exit aperture D₂, at least for thisparticular example.

As shown in FIG. 3, a short Beam Shaping Assembly (BSA) in the shape offunnel 222 is provided after Ti target 106 and the short moderator orpre-moderator 108, where the thermal neutrons may be collected by alarge aperture D₁ and directed to a smaller aperture D₂ at the end offunnel 222. In a simple embodiment, funnel 222 is an inverted conemachined into the BSA support structure (or plate) 234 made of HDPE asshown in perspective view FIG. 3 with an entrance aperture D₁ and anexit aperture D₂. Cone-shaped funnel 222 in this embodiment is formedinto the plate 234 of HDPE. Other materials such as Teflon, UHMWpolyethylene, or graphite can be used for the plate. As shown in FIG. 3,an exit aperture 250 is also machined in the shielding 248 to define thethermal neutrons coming from the exit aperture.

After collection at aperture D₁, the resulting thermal neutron beamexits at aperture D₂, providing an increased flux and smaller sourcesize for the thermal neutrons when compared to a simple pre-moderator.

FIG. 4A is a plot of flux density vs. lateral dimension in cm. at D₂ inone example. In the example of FIG. 4A, the spot size is roughly 1.5 cmat diameter (D₁). Without the funnel 222 the diameter of the source sizewould be a disc 6 cm in diameter and expanding (FIG. 4). The use of themoderator in direct or near contact with Titanium target 106 is toproduce a maximum thermal neutron flux at the top aperture D₁ of thefunnel 222. Adding a funnel 222 of modest length (L₂=4 cm in thisexample) directly on to the pre-moderator permits a maximum number ofneutrons and higher thermal neutron flux to be collected and a smallersource size to be obtained. The close stacking of the pre-moderator 108and the BSA (Funnel 222 in HDPE slab 234) permits a maximum number ofthermal neutrons to be obtained with a small source size. FIG. 4Aclearly illustrates the beneficial effect of the slab with the funnel.

To reduce the size of the thermal neutron beam emitted by the HDPEmoderator at its maximum thermal neutron flux (n/sec-cm²), funnelaperture 222 is added along the axis of the generator 118 as defined bydirection of the D⁺ ion beam 104, and the titanium target 106. As shownin FIGS. 3A and 3B, the dimensions of the apertures in this example are:D₂=1.5 cm, D₁=6.0 cm, L₁=4.0 cm, L₂=3.0 cm., E<0.5 eV. The plot in FIG.4 is from a Monte Carlo Neutral Particle (MCNP) code simulation forthese parameters. For the case with no funnel, the moderator shows abeam spot of FWHM of 10 cm in diameter. With the funnel 222 in place,the FWHM of the emission is roughly 2 cm in diameter, defined by a knowntechnique called Full Width Half Maximum (FWHM) and the neutron flux is2 times larger than an uncollimated neutron beam.” The source size afterthe funnel has a full width half maximum of 2 cm, whereas forpre-moderator only, it is 10 cm. One can achieve a high resolution ifthe source is at an enough distance L, and small enough emission sizediameter, D₂, to have a small D₂/L. This permits a resolution ofΔx(D₂/L) for an object that is Δx=1 mm thick. For a D₂ of 1.5 cm and anL=20 cm, the resolution is 0.1 mm.

A prototype of the apparatus has been built and tested at the time offiling the present patent application. The apparatus is shown in FIGS.3A and 3B with a funnel collimator as shown in the two figures, asdescribed above. The dimension of the prototype funnel collimator areD₁=6 cm and D₂=1.5 cm with L₁=5.5 cm and L₂=4 cm. These dimensions maybe different in other embodiments. The method of detection of thethermal neutron is a linear array of small chips of NaCl. These chipswere activated by the in-coming thermal neutrons for a measured lengthof time. The radioactive flux was then determined by neutronspectrographic means. The results are shown in FIG. 4B. The generalshape and magnitudes are comparable with the simulated results of FIG.4A. The peak flux of FIG. 4B is smaller (6×10⁶ n/sec-cm²) than that ofthe simulation of FIG. 4A. (9×10⁶ n/sec-cm²). The differences are fromthe resolution of the detectors (measured vs simulated). The simulationof FIG. 4A used a shorter pre-moderator, L₁=4.0 cm. vs L₁=5.5 cm.However, in both case L₂=5.5 cm and the comparison between 4A and 4Bshows that the HDPE funnel BSA is indeed effective as a method ofcollimation.

In examples of BSAs, convergent collimators are used. Other geometriescan be used such as divergent collimators, which reverse the directionof the truncated cone. These have been used throughout the nuclearreactor source industry. Some collimators have a divergent-convergentshape, which can result in a shorter BSA length and higher thermalneutron flux.

To see how well the conical aperture source performs, an image of a 1.0mm thick Gadolinium (Gd) knife edge 238 is simulated, placed in front ofa detector array 224 made of 5.0 mm of H₂O, 142, as shown in FIG. 5. Forthis calculation, the desirable image properties are (a) a flux ofgreater than 10³ n/(s-cm²), and (b) a desired resolution is 1 mm, withthe contrast of an order of magnitude or greater.

To see how well the conical aperture neutron source works, an image of a1.0-mm-thick-Gadolinium (Gd) knife edge 238 with a conical BSA 222 issimulated. Water (5-cm, 240), is used to simulate materials in thedetector 246 which scatter the thermal neutrons. The Gd knife edge 238is placed on the upstream side and in front of the H₂O, 240 to determineresolution and contrast. With the arrangement shown in FIG. 5, L is thedistance between the BSA 222 and the Gd-knife edge 238. In all thesimulations shown in this submission, a diode array detector 246 isassumed to be at a distance of 1.0 mm from the back of the H₂O 240. TheBSA 222 is an air cone embedded (machined in HDPE with an entranceaperture of D₁=6 cm and an exit aperture of D₂=1.5 cm). The neutron flux(n/cm²-sec) is found using a Monte Carlo Neutral Particle (MCNP)simulation shown in FIG. 6, which compares the thermal (E<0.5 eV) andfast (E>0.5 eV) energy neutron flux components.

To attempt to achieve these properties, various modulator and BSAarrangements are considered. The object is a 1.0-mm-thick Gd knife edge,238, backed by 5-mm of H₂O, 240, and is placed on the upstream side ofthe H₂O to determine resolution and contrast. In the generator, the ionbeam 104 strikes a 5-cm diameter Ti target 106 and 2.5-MeV neutrons areemitted into the pre-moderator of thickness L₁=4 cm. FIG. 6 shows thesimulated Monto Carlo in Transport (MCNP) thermal flux 244 and fast 242neutron fluxes as a function of x across the detector array 224. Forthis arrangement, the resolution is calculated to be 8.5 mm. Othersimulations with somewhat different parameters gave resolutions of 3.3mm.

The fast neutrons created from the moderation process are shown in thetop curve in FIG. 6. Note, the fast neutrons 242 in the simulation arenot being contrasted across the Gd knife edge. Indeed, most neutrondetectors cannot easily distinguish between fast and thermal neutrons.However, separation between the fast and the thermal neutrons can beachieved by pulsing the neutron beam. The generator can be modulated bypulsing the ion beam, interrupting the microwave power that is creatingthe D⁺ ions. Pulses as short as 10 μsecs have been produced by theinventors using this method. The fast neutrons are created at thetitanium target and then pass through 4 cm of the HDPE pre-moderatorwhere approximately 50% are reduced to thermal energies. For a 10 μsecpulse of neutrons, the difference in speed (2.2 km/sec for thermals and1.4×10⁴ km/sec for fast neutrons) results in thermal neutrons lagging by20 μsec when they reach the detector at 20 cm from the aperture. Timingof the reading of the fast and thermal neutron pulses detected by acharge-coupled device (CCD) camera allows to distinguish between the twoimages; one caused by the thermal neutrons and the other by the fastneutrons. The neutron source properties are sufficient neutron flux(e.g. 10³ n/(s-cm²) or greater coming from a small spot size. Millimeterresolution is desired having an image contrast of an order of magnitudeor greater.

With the arrangement in FIG. 5 and L=2.0 cm, FIG. 6 shows thermal andhigher energy neutron flux. For these parameters, the flux is good, thecontrast is good, the transverse dimension is good, but the resolutionis ≈8.5 mm.

To improve the resolution, different parameters for D₁ and D₂ areselected, and different distances from the BSA aperture D₂ 228 to theknife edge are tried, L. All other parameters for the generator, knifeedge and detector array are the same. Larger apertures D₂=5 cm, and D₁=8cm. The distance to the knife edge L=2 cm. Flux as a function of x(cm)is shown in FIG. 7A. The maximum thermal flux is a healthy 3.4×10⁶n/cm²-sec, but the resolution is 6-mm. Increasing the distance L to 100cm, as shown in FIG. 7B, sub-mm resolution is achieved. Throughout thesesimulations, HDPE is used in both the moderator 108 and in the BSA 222,the thickness of the Moderator is L₁=4 cm, and the thickness of the BSAis L₂=4 cm. Plotting the resolution as a function of L in FIG. 8, theresolution continues to improve. We can achieve the desired resolutionsof 1 mm for L=20 to 50 cm.

However, with increasing distance L, the available neutron flux forimaging decreases resulting in an increase in measurement time for thecollection of neutrons. This may be estimated with a simple assumptionthat each diode of the array needs around 250 neutrons for ameasurement. Plotting the measurement time in FIG. 9 as a function of L,we see that we achieve measurement times in the order of seconds. WithL=20 to 50 cm, the detection of the knife edge image with 1-mmresolution can take place within 1 to 3 seconds, a time more thanadequate for achieving a high-quality image.

Convergent collimators are used throughout this submission, butdivergent collimators or combinations of both convergent and divergentcollimators may also be used. In the divergent conical collimator, thecone may be lined with grazing angle reflective materials such asCadmium, Indium, B₄C or Boron. The cone is made of a machinable materialand lined with Cd, In, or B. The use of HDPE without a lining (Cd, In,or B) in the convergent collimator, as taught in this disclosure anddemonstrated by MCNP simulation, ensures that both collection andfurther moderation of the neutrons to thermal energies can be achieved.

The resolution may further be improved by attenuating the fast neutronsby means of a low pass filter, in which thermal neutrons aretransmitted, while fast neutrons are attenuated. Fast neutrons need tobe attenuated or the detector's sensitivity to the fast neutrons needsto be suppressed. To eliminate fast neutrons, a 9-cm long sapphirecrystal may be added to the BSA. Sapphire (Al₂O₃) is an effectivefast-neutron filter because its transmission for neutrons of wavelengthsless than 0.04 nm (500 meV) is less than 3% for a 100 mm thickness.

Current technology enables large diameter, single-crystal sapphireingots to be grown using what is known as the Kyropoulos technique.Diameters of sapphire ingots may be 5 to 12 cm with thickness of 5 to 20cm long. It is estimated that 7 cm of sapphire may reduce fast neutronyield by an order of magnitude while transmitting roughly 80% of thethermal neutron flux.

FIGS. 10A and 10B show a modular generator with many of the sameelements as shown in FIG. 3, but with additional detail of HDPErectangular plate 234 and elements in a passage through plate 234 forfiltering a neutron beam before the collimating funnel 222.Pre-moderator 108 using ˜5.5 cm of HDPE produces in this example aneutron beam that is roughly equal in fast and thermal neutrons. Asshown in FIGS. 10A and 10B a sapphire crystal 220 and a Bismuth filter238 may be added that reduce the number of high energy gamma raysproduced by thermal neutron capture of hydrogen in the HDPE. Sapphirecrystal 220 in this embodiment may be a cylinder with an outer diameterfitting into sleeve 236, between 5 to 15 cm long with a high enoughaverage atomic number, Z, to attenuate the gamma background. Bismuthfilter 238 is a disk of the diameter of the sapphire crystal fittinginto sleeve 236 just above the sapphire crystal. Sapphire filter 220 isadded just below the pre-moderator 108 and into a sleeve 236 whichencloses the sapphire filter 220, acts as a reflector and attenuates theneutrons that are outside the sapphire crystal 220. A funnel 222 whichis L₂=4 cm long and a reducing aperture D₁ which directs the thermalneutrons to the desired minimal aperture D₂ for a desired small beamdiameter D₂. It is important to minimize the distance (L₁+L₂+L₃+L₄+L₅)from the Ti target 106, to the aperture D₂, 250. This maximizes thethermal neutron yield delivered to the small aperture D₂, whileminimizing spurious radiation of gammas and fast neutrons. Thismaximizes the number of thermal neutrons required for good imageresolution and contrast.

The D⁺ ion beam 104 strikes the titanium target 106, where D⁺ ions areembedded and creates the DD fusion reaction, resulting in the isotropicemission of fast (2.5 MeV) neutrons. To maximize the flux beingtransmitted through the sapphire filter, the crystal 220 is aligned withits axis in line with the ion beam 104 direction and the maximumincoming thermal neutron beam. The sapphire crystal length andorientation is selected to maximize the thermal neutron transmissionpreferably in a wavelength range of 1.2 to 2.5 A, while minimizing fastneutron wavelengths of less than 1 Angstrom. Fast neutron transmission,T, decreases exponentially with crystal length, L: orT=I/I_(o)=exp(−L/L_(o)). In this embodiment a sapphire crystal length of70 mm is selected, which roughly gives an order of magnitude decrease inthe fast neutrons relative to the thermal.

Assuming a mixed neutron beam is being transmitted thru the Sapphirefilter, it is desired to maximize the thermal neutrons while suppressingthe fast and epithermal neutron components and the gamma rays producedin the HDPE pre-moderator material. It is desired in this example tomaximize transmission of a 2.5 cm beam, defined by the definition ofFull Width Half Maximum (FWHM), of thermal neutrons down a cylinder 220composed of Sapphire crystal. Thermal neutrons are being scatteredduring transmission and some are lost outside the sapphire crystal.However, a sleeve 236 of high density (or high Z) reflecting materialjust outside the crystal surface may reflect the thermal neutrons backinto the crystal and thereby increase the total neutron yield at theexit to the BSA. In this example the sleeve 236 is Bismuth surroundingthe sapphire crystal. The high Z sleeve critical angle reflects anygrazing-angle thermal neutrons but scatters and absorbs the higherenergy neutrons that pass from the sapphire to the Bismuth. Ideally, thethermal neutrons travel down the sapphire cylinder and the fast neutronsget absorbed or scattered. The conical aperture at the end of theSapphire crystal acts to transmit the thermal flux out a small aperture(D₂). The conical aperture 228 diameter tapers from D₁=6 cm to D₂=1.5cm. in this implementation There are other parameters and materials,such as graphite, that can be used to form the conical aperture 222 andthe rectangular plate 234.

Because tungsten target 106 is on the plastic (HDPE or Teflon)pre-moderator 108, fast neutrons coming from the target immediatelyenter the pre-moderator and can be moderated to thermal or epithermalenergies. A short Beam Shaping Assembly (BSA) is provided below the Titarget and the pre-moderator, where some of the thermal neutrons may becollected and directed to a small aperture at the end of the BSA. Ashort, L₅, iris 250 is placed just below the BSA. The material of theiris 250 may be made of lead and B₄C. In its simplest embodiment, theBSA is an inverted cone 222 as shown in FIGS. 3A and 3B. The HDPE of theBSA acts as a reflector and collimator of the thermal neutrons. Asstated elsewhere, the collimator can be made of other materials such asgraphite. The resulting thermal neutron beam at the aperture of the BSAgives a higher flux and smaller source size for the thermal neutronswhen compared to a simple moderator. In the example shown, the spot sizeis roughly 1.5 cm in Diameter (D₂). Without the BSA the diameter of thesource size at the pre-moderator would be a disc 6 cm in diameter andexpanding. The idea behind the use of the pre-moderator 108 in direct ornear contact with Titanium target 106 is to produce a maximum thermalneutron flux. Adding a BSA of modest thickness (L₂=4 cm in this example)directly on to the pre-moderator permits a maximum number of neutronsand higher thermal neutron flux to be collected and a smaller sourcesize to be obtained. The close stacking of the pre-moderator and the BSApermits a maximum number of thermal neutrons to be obtained with a smallsource size D₂.

In embodiments of the invention, thermal neutron collection can beachieved with a conical funnel to both collect and channel neutrons intoa small spot size with increased thermal flux at the exit of the cone ofthe funnel. The compact DD fusion source with a short thermal moderator(such as HDPE, or UHMW plastics with a high concentration of hydrogenatoms) quickly scatters the fast 2.5 MeV neutrons to thermal energies ina short distance (L₁+L₂+L₃+L₄+L₅) from the fast neutron source (thetitanium target 106). As shown in FIGS. 3A and 3B, the neutrons are thencollected by a relatively short (e.g. L₂=3 to 4 cm) funnel 222 in theslab of HDPE. Both moderation and scattering continue to occur along thecone length L. Simulations show that this results in an increase in flux2 to 3 times and spot sizes of 1 to 3 cm in diameter depending on thegeometry of the cone and size of the fast neutron emitter (Ti target).Additions of short spatial and energy filters improve the image byimproving the brightness of the neutron source and limiting the effectsof spurious radiation of fast neutrons, gamma emission.

In embodiments of the invention, the shortness and compactness of the DDfusion generator and the moderation process to produce and collectthermal neutrons also allows for the use of other devices in the beamincluding short lengths of sapphire 220 and bismuth crystals 240, whichcan reduce the fast neutrons and gamma emission in the neutron beam,thus cleaning up the beam and achieving a relatively pure beam ofthermal neutrons. The use of a compact fusion generator with relativelysmall spot sources of neutrons permits these neutron filters to also becompact and close together. This results in a useful source of neutronsthat can be used in many laboratories and field locations, unlike thefixed, large and expensive reactor sources.

The invention claimed is:
 1. A neutron radiography generator,comprising: a modular neutron source generating an ion beam in a firstdirection, the ion beam bombarding a Titanium target having a surfacecomprising a first diameter, the target embedded in a pre-moderatorhaving a lowermost surface, emitting fast neutrons isotropically, aportion of the fast neutrons moderated in passing through thepre-moderator and exiting through the lowermost surface of thepre-moderator; and a plate of moderating material abutting the lowermostsurface of the pre-moderator, the plate having an upper surface, a lowersurface, a thickness, and an opening therethrough in a shape of atruncated cone with an axis aligning with the first direction of the ionbeam, a depth, a major diameter of at the upper surface of the plate anda minor diameter at the lower surface of the plate, the opening forminga funnel through which neutrons pass; wherein neutrons exiting thepre-moderator through the lowermost surface thereof enter the funnel andare collimated through the funnel to exit through the minor diameter ofthe funnel, providing a neutron beam with a spot size useful for neutronradiography.
 2. The neutron radiography generator of claim 1 wherein thethickness of the plate of moderating material is greater than the depthof the funnel, further comprising a sleeve of high-density neutronreflecting material having in inside diameter equal to the majordiameter of the truncated cone shape of the funnel, the sleeve lining acircular hole in the plate of moderating material, the hole having afirst depth from the upper surface to the major diameter of thetruncated cone shape of the funnel, such that the depth of the circularhole and the depth of the funnel equals the thickness of the plate, andthe inside diameter of the sleeve of reflecting material meets the majordiameter of the truncated cone shape of the funnel.
 3. The neutronradiography generator of claim 2 further comprising a disk of Bismuthhaving a diameter equal to the inside diameter of the sleeve and alength less than the first depth of the hole, and a solid cylinder ofsapphire crystal having a diameter equal to the inside diameter of thesleeve and a length such that the length of the bismuth disk and thesapphire cylinder equal the length of the hole from the upper surface ofthe plate of moderating material to the major diameter of the funnel,the bismuth disk and sapphire crystal attenuating gamma radiation andattenuating fast neutrons.
 4. The neutron radiography generator of claim1 further comprising a layer of shielding material cladding the outersurfaces of the generator including the lower surface of the plate ofmoderating material, with a hole through the shielding material at andequal in diameter to the minor diameter of the funnel.